Substation telephone circuits



A. B. SMITH SUBSTATION TELEPHONE CIRCUITS Original Filed Jan. 5, 1924 Patented Oct. 23, 1928.

ARTHUR BESSIE! SMITH OF EVANSTON, ILLINOIS,

PATENT OFFICE.

ASSIGNOR, BY MESNE ASSIGN- MENTS, TO AUTOMATIC ELECTRIC INC., OF CHICAGO, ILLINOIS, ACORPORATION 0] DELAWARE. I

SUBSTATION' TELEPHONE CIRCUITS.

Application filed January 5, 1924, Serial No. 684,483. Renewed July 5, 1928..

The present invention relates in general to substation telephone wcircuits, but more especially to substation telephone circuits for automatic telephone systems; and the object of the invention is to provide a new and improved local battery anti-side tone circuit for an automatic telephone. The circuit, as

illustrated herein, is especially designed for a desk telephone, but the principles of the invention'are applicable also to a wall tele-- phone.

Having set forth the object and scope of the invention in general terms, I will now explain the circuits and apparatus involved more in detail, reference being had to the accompanying drawing, in which Fig. 1 is a wiring diagram of a local battery automatic desk telephone and bell box; Fig. 21s a simplified circuit diagram; while Fig. 3 is a schematic diagram of the anti-sidetone'talking circuit.

Referring first to Fig. 2, the talking C11- cuit as before stated is of the well' known local battery type. With the receiver removed from the switch-hook, the transmitter T, the battery B, and the winding 6 of the induction coil I are included in a local 011- cuit while the secondary windings' t and 5 of the induction coil are bridged across the line conductors 2 and 3 in series with the balancing resistance 7. The receiver R, in series with condenser G, is connected between the line conductor 2 and the junction of induction 'coil windings 4 and 5. The transmitter and receiver circuits are normally open and are closed in the usual manner by the swltchhook H when the receiver is removed. Contact springs 10 and 11 are included inthe receiver circuit, while contact springs 8 and 9 are included in the transmitter circuit.

For receiving incoming signals the ringer M is provided, which is normally bridged across the line independent of control by the switch-hook and is in series with the condenser C'.

Associated with the lower side of the line or calling. device, which is indicated by the reference character S. This sender may be of the type which is'shown and describedon pages 34 and 35 of Automatic Telephony by Smith and Campbell, published by the McGraw- Hill Book Company. It comprises essentially a pair of impulse springs 15 and 16 .not in use,

which are controlled by a cam (not shown) throu h the medium of a finger hold dial (not s own) and three shunt springs 12, 13 and 14, together with suitable means for retalmn these springs in the position shown in the rawing when the calling device is and for causing all three springs to come mto engagement with each other when the calling device dial is turned from 1ts normal position. The impulse SEI'HIgS 15 and 16 are connected in series with t e lower line conductor 2 and their function 1s to produce momentary interruptions of the line circuit for the purpose of operating the central ofiice switches. The function of the shunt springs 12, 13 and 14 is to short circuit or shunt the receiver R and the balanc ng resistance 7 while the calling device is being operated to transmit impulses. To accomiplish this spring 12 is connected to one slde 0 the receiver, spring 13 is connected to the junction of the compensating resistance and winding 5 of the induction coil, while spring 14 is connected to impulse spring .15 which in turn is connected to the other side of the receiver, and it will be evident that with the sprin s connected in this manner, when they-are rought into engagement the receiver and reslstance 7 will be short cirproper; while the apparatus at the left of the drawing and comprising the ringer M, the

condensers O and C, and the induction coil I is separate from the telephone and in ac-' *cordance with th e usual practice is located in the bell box. The telephone is provided with a terminal s'trip P in the base thereof, and the bell box is provided with a terminal strip P. The various circuits are completed by means of a cord which connects the two terminal strips, and which, it will be observed, comprises five conductors. The line extending to the exchange is connected at the terminal strip P as illustrated in the drawing.

The operation of the substation circuit is as follows: When the receiver is removed to of the line switch,

- initiatd a call, the switchhook contact'springs 9 and 10 come into engagement with contact springs 8 and 11, respectively, whereby the transmitter'and receiver circuits are closed and the secondary windin of the induction coil is bridged across the line in series with the resistance 7. Assuming that the line terminates at'the exchange in the usual line switch, the closure of t e bridge at the substation completes the circuit of the line relay which therefore operates to extendtthe line to a numerical switch or first selector. The calling subscriber now manipulates the calling device dial to produce a series of interruptions for directively controlling the 'first selector by means of its line relay.

Consi ering{a little more closely the operations which ta e place at the telephone during the transmission, of a series of impulses, it will be noticed that as soon as the dial is turned from its normal position the shunt s rings are closed together,

be traced from the line conductor 3 by way of windings 4 and 5 of the induction coil I, shunt prin s 13 and 14, impulse springs 15 and 16,

s and the switchhook springs 11 and 10 to the device dial returns to its normal position. At

' the same time, the receiver R is short circuited by way of shunt springs 12, 13 and 14. This latter operation takes care of the usual requirement of silent operation; in other words, with the receiver short circuited the subscriber is not annoyed by hearing the clicks in the receiver, caused by'the interruptions in the line circuit.

To continue, the first operation of the calling device brings about the operation of a first selector switch at the exchange, and succeeding operations of the calling device will whic with the exception of t now resu t in the operation of additional switches until the connection'is finally comleted to the line of the called subscriber, it

eing understood that all of the switches are rovided with controllin relay brid es e bridge in t e last switch, are successively cut ofl from the line after their associated switches are operated.

The connection having been completed, conversation may be held as desired, the talking circuits functioning the same as usual in such cases, except that the side tone in the subscribers receiver is eliminated. or greatly reduced when he talks in the transmitter, as will be more fully explained hereinafter. The receiver is replaced at the end of the conversation, the transmitter and receiver circuits are and the substation loop is completed over a path which may course,

broken at the switchhook contacts, and further flow of direct current through the substation is prevented. As a result all the switches used in the connection at the central ofiice are released and restored to normal in position and the receiver is ofl the switch- 00k; in other words, while conversation is being carried on. Since all the non-essential elements, such as the calling device, ringer, etc., are omitted in the schematic drawing, the relation between the parts shown may be per.- ceived at once. It Wlll not be necessary to go into a minute consideration of the operation of the circuit from a strictly technical stand point, but it may be stated, however, that when the subscriber talks in the transmitter T and produces variations in the direct current flowing in the local circuit including the battery B and primary winding 6 of the induction coil I, varying electromotive forces arise in the secondary windings 4 and 5 and produce varying currents in the secondary circuits which may be considered as comprising the two side circuits of the three wire system. One of the side circuits is composed of the line conductors 2 and 3 with the receiver, etc., (not shown) at the distant end, the winding 4, condenser C, and receiver R; while the other side circuit is composed of the compensating resistance 7, winding 5, condenser C, and receiver R. It will be observed that lid:

the receiver R and condenser C are in a con common or neutral conductor, and that if the compensating resistance 7 is equal to the loop resistance of the line or, in other words, if the side circuits are roperly balanced, practically no current wi l flow in the common or neutral conductor. Therefore it willbe seen that, due to no current flowing throu h the receiver R, no side tone will be heard by the calling arty. Incoming voice currents, of ivide at the substation, part flowing through the branch including the resistance 7 and a substantial part flowing through the receiver branch.

Compensating resistance 7 used is of a value approxnnating that of the average loop resistance and this has been found to be sufficiently accurate for all practical purposes, although in special cases the value of the resistance may be more exactly adjusted, and it may be supplemented by a capacity and inductance in order to closely balance the line. The resistance may be dispensed with if its value is added to the winding 5 of the induction coil I. 1

While the explanation might perhaps be further amplified, it is believed that the foreoing will suffice to enable the invention to be understood and used by those skilled in the art. It will be seen that I have devised an especially advantageous circuit for a local battery automatic desk-telephone which includes transmitting and receiving'circuits of good efficiency and practically eliminating side tone in the talking partys receiver.

Having described my invention, what I consider to be new and desire to have protected by Letters Patent will be pointed out in the appended claims. What I claim as my inventionis: 1. In a substation telephone set, a transmitter and local circuit therefor, an induction coil. having a primary and two secondary windings the primary winding being connected in said local circuit, a resistance, a

bridge across the two lme conductors 1nclud-.

ing in a. series circuit said secondary windings and said resistance, a receiver and a condenser in series connected between one line conductor and the junction of said secondary windings, and a calling device having a pair of interrupter springs in series with one of said line conductors.

2. In a substation telephone set, a transmitter and local circuit therefor, an induction coil having a primary winding connected-in said local circuit and two secondary windings. a resistance, a bridge across the two line conductors including said secondary windings and said resistance, a receiver and a condenser connected in series between one line conductor and the junction of said secondary windings, a calling device having a pair of interrupter springs in series with one of said line conductors, and shunt springs for short circuiting said resistance while the calling device is being operated.

3. In a substation telephone set, a transmitter and local circuit therefor, an induction coil having a primary and two secondary windings the primary winding being connected in said local circuit, a resistance, a bridge across the two line conductors including in a series circuit said secondary windings and said resistance, a receiver and a condenser in series connected between one line conductor and the junction of said secondary windings, a calling device having a pair of interrupter springs in series with one of said line conductors, and shunt springs for short circuiting said receiver while the calling device-is being operated.

4. In a substation telephone set, a transmitter and local circuit therefor, an induction coil having a primary winding connected in the resistance of said local circuit and two secondary windings, a resistance, a bridge across the two line conductors including said secondary windings and said resistance, a receiver and a condenser connected in series between one line conductor andthe junction of said secondary windings, a calling device having a pair of i terrupter springs in series with one of said ine conductors, and shunt springs actuated while the calling device is being op- 7 erated for short circuiting said resistance and j I the receiver;

5. In a substation telephone set, an induction coil having a primary and two secondary windings, a transmitter in a local circult mcludin said primary windin a resistance included in a series circuit wit said secondary windings and bridged across the two line conductors, a receiver and a condenser in series relation and connected between'one 35 line conductor and the junction of said secondary windings, a calling device having a a pair of interrupter sprin normally in a series circuit including one Ii ne conductor, the secondary windings of said induction coil and said resistance, and shunt springs for short circuiting said resistance during the operation of the calling device.

6. In a substation telephone set, a line comprising two conductors leading to a central exchange, an induction coil' having a primary and two secondary windings, a trans-. mitter in a local circuit including said primary winding, a resistance approximating.

said line included in series relationwith said secondar windings and bridged across said line, said resistance together with one of the secondary windings of said induction coil forming one side of a Wheatstone brid ,while'the other side is represented by the ine resistance and the other of the secondary windings of said inductioncoil, and a receiver in series with a condenser connected ,to the neutral points of said Wheatstone bridge.

7. In a substation'telephone set, a two conductor line leadin to a central exchange, a resistance up roxlmating the ohmicresiste ance of said 'ne, a calling device having a plurality of springs actuated durin eration for directively controlling t e apparatus in; the central exchange over a path including two secondary windin s of an induction coil bridged across sai line, said springs producing circuit changes cessation of the operation of said calling de-" vice, adapting said setto transmit and'to re ceive talking currents over a path involving said secondary windings in series with said resistance bridged across the windings together with said resistance forming one side of a Wheatstone brid e, while the other side is represented by said ine and the other of said secondary windings, a receiver in series with a capacitance being connected. 1

its op- 116' upon the b line, one of said i to' the neutral point of said Wheatstone bridge. 8. In a substationset, a line circuit, a'transmitter, a receiver, a calling device having im- 6 pulse s rings and shunt springs, a transformer aving-three windings, a bridge across said line circuit including one winding of said transformer and said impulse springs, a local circuit for said transmitter including a second winding of-said transformer, a local circuit for said receiver including a third winding of said transformer, a conductive coupling between said second and third windings, and circuit means connecting said shunt springs to maintain said substation at a constunt resistance under the control of said impulse springs during the operation of said calling device.

In witness whereof, I hereunto subscribe my name this 3d day of January, A. D. 1924.

ARTHUR BESSEY SMITH. 

